Fix double nspawn

*.container hooks were not working inside another nspawn because it tries to find a system dbus and fails:

P: Executing hook config/hooks/live/enable-flathub.container... Spawning container chroot on /mnt/usb-live-linux/build/chroot. Press Ctrl-] three times within 1s to kill container. Failed to open system bus: No such file or directory E: config/hooks/live/enable-flathub.container failed (exit non-zero). You should check for errors. E: An unexpected failure occurred, exiting... P: Begin unmounting filesystems... P: Saving caches...

c.f. https://bugs.archlinux.org/task/55082 :

This is probably caused by the fact that systemd-nspawn now registers a scope when --register=no is specified, unless --keep-unit is also specified [1]. [1] https://github.com/systemd/systemd/pull/6166